Peavey Classic 30 Hum

[Billy Penn]

I'm sure this has been posted before and I did a search but could not find the solution. So here goes... Peavey Classic 30 that hums. Amp functions 100% and sounds good. I tried replacing all tubes and the hum is still there. Here's what I know so far:
1- Reverb on zero= Hum still there.
2- Remove preamp tubes= hum still there.
3- Plug guitar into effects return= hum slightly decreases.
4- Use jumper in effects loop= hum still there.
5- Remove output tubes= hum goes away.
I'm thinking bad filter cap for plate or screens? Not sure if it's the bias cap because it does not hum with the output tubes removed. I read about a similar issue with a bad JP520 jumper but this one seems intact. Does anyone have any experience with this amp and a cure for the hum? Thanks in advance!!

[gearhead63]

I would start by checking the power supply and see how much ripple there is on your DC voltage.
Then go from there. Always check power first! You can check for ripple on the high voltage to the tubes too.
Can you tell if it's a 60hz or 120hz hum?
And do you have a scope? Makes it easier, but you can use a VOM.

[nosaj]

Did you verify the jumper with a meter or just visual?
nosaj

[Billy Penn]

No scope but do have a VOM. Not sure if it's 60 or 120....I think 120. It sounds like when tubes are going to red plate but all voltages and current are good.

I would start by checking the power supply and see how much ripple there is on your DC voltage.
Then go from there. Always check power first! You can check for ripple on the high voltage to the tubes too.
Can you tell if it's a 60hz or 120hz hum?
And do you have a scope? Makes it easier, but you can use a VOM.

[Billy Penn]

I used a meter. I will triple check it again tomorrow. Thanks!!

Did you verify the jumper with a meter or just visual?
nosaj

[nosaj]

When you pulled the preamp tubes did do them one at a time or pull them all at once?
nosaj
Also found this download the wave file to hear the sound.

Hum samples 60Hz vs. 120Hz & Seeburg trip -new link 9-23-15
When diagnosing an amplifier hum problem the nature of the noise can be helpful. So here are two typical examples.

This is a .wav approx 23 seconds of hum. First is the ever-popular 60Hz followed by 120 Hz.

The first was from an ungrounded preamp input and has the usual "snivets". The second is from a generator.

In listening to these it is best not to use too high a volume on small speakers as this can cause harmonics and confuse things.

60Hz Hum is typically from inadequate shielding, heater-cathode leakage and ground loops. Inadequate half-wave filtering can also cause this.

120Hz Hum is only found on full-wave power supplies when filtering is inadequate or the load exceeds the time-constant of the filters.

Hopefully these will make diagnosing amp hum problems a little easier.

[Billy Penn]

I removed the preamp tubes one at a time and all three at once.

I listened to the sample but it seems that there is only one hum and about half way through it cuts off. The hum I heard on the file does not sound anything like the hum in the amp I have. The hum sounds like the output tubes are going to red plate but never do.

When you pulled the preamp tubes did do them one at a time or pull them all at once?
nosaj
Also found this download the wave file to hear the sound.

Hum samples 60Hz vs. 120Hz & Seeburg trip -new link 9-23-15
When diagnosing an amplifier hum problem the nature of the noise can be helpful. So here are two typical examples.

This is a .wav approx 23 seconds of hum. First is the ever-popular 60Hz followed by 120 Hz.

The first was from an ungrounded preamp input and has the usual "snivets". The second is from a generator.

In listening to these it is best not to use too high a volume on small speakers as this can cause harmonics and confuse things.

60Hz Hum is typically from inadequate shielding, heater-cathode leakage and ground loops. Inadequate half-wave filtering can also cause this.

120Hz Hum is only found on full-wave power supplies when filtering is inadequate or the load exceeds the time-constant of the filters.

Hopefully these will make diagnosing amp hum problems a little easier.

[nosaj]

turn your volume up a little more its a lower sound. Don't max it but turn it up. I didn't hear it at first either

nosaj

Here's one from Uncle Doughttps://www.youtube.com/watch?v=GrVtX0QGNls

[Billy Penn]

The hum in my amp is definitely not 60hz. Sounds a lot more like 120hz for sure!!

turn your volume up a little more its a lower sound. Don't max it but turn it up. I didn't hear it at first either

nosaj

Here's one from Uncle Doughttps://www.youtube.com/watch?v=GrVtX0QGNls

[Enzo]

You can't remove the tubes one at a time. Well, you can mechanically, but their heaters are series wired, so removing one preamp tube makes all three go dark. Likewise, pulling one power tube turns off all four of them. Pulling one or all four power tubes means the amp has NO WAY to make anything come out the speaker.

You do need to find out if any B+ supply is rippling or the bias supply as well.

Removing a preamp tube kills the preamp, so if hum remains, then the power amp has the hum.


And make sure all the jack nuts are snug as well as the control bushing nuts. The jacks are part of the board grounding systems.

[xtian]

I would try two simple and quick tests. Try a new, matched set of power tubes. If one is failing, that can cause hum.

Second, clip in a 47uF/450v electrolytic at B+ and see whether that silences the hum. In Fender's Hot Rod series, for example, the reservoir caps are a very common hum-causing failure.

[Billy Penn]

I did try a new quad of EL84's and did not cure the hum. I will try clipping in a 47uf/450v cap at the B+. I have a feeling it's either a bad B+ or screen cap. Thanks for the reply and I will post back when I do this test.

I would try two simple and quick tests. Try a new, matched set of power tubes. If one is failing, that can cause hum.

Second, clip in a 47uF/450v electrolytic at B+ and see whether that silences the hum. In Fender's Hot Rod series, for example, the reservoir caps are a very common hum-causing failure.

[Billy Penn]

I replaced the two 47uf/500v B+ caps, 22uf/500v for the screens and the 22uf/25v bias cap. Also while I was under the hood I re-flowed all the jumper solder joints. With output tubes installed I still have the hum. The only other thing I can think if is possibly the two 2200uf/50v caps that feed the bias circuit. Any other thoughts from anyone before I order the 2200uf caps..?? Thanks!!

[Jazz P Bass]

It would help if you actually measured the Volts ac ripple on the capacitors.

Simply changing them out does not prove nor disprove anything.